A 45-year-old woman presented with a 10-day history of an erythematous, scaly rash on her back that developed after a period of intense sun exposure. She did not have a fever or chills, and there was no discharge from the site. The patient was otherwise healthy. She had a history of small, erythematous, scaly lesions on both elbows during adolescence that spontaneously resolved. Her family history was significant for maternal psoriasis.

Physical examination revealed multiple erythematous, scaly plaques on the lower back just above the waistline (Figure 1). These lesions were in the area of sun exposure. There were no other lesions or physical findings.

Question

Based on the patient's history and physical examination findings, which one of the following is the most likely diagnosis?

A. Herpes zoster.

B. Irritant contact dermatitis.

C. Koebner phenomenon.

D. Pityriasis rosea.

E. Tinea corporis.

Discussion

The answer is C: Koebner phenomenon. Koebner phenomenon is an isomorphous rash that is commonly associated with psoriasis and other skin diseases such as vitiligo and lichen planus.1 It occurs following local trauma in areas that are not involved with skin disease.2 The rash appears as clearly defined, erythematous, scaly plaques.3 Provoking factors can include physical trauma, friction, surgical incision, burns, and radiation exposure.4 Koebner phenomenon appears to be unrelated to the activity or severity of the associated skin disease.5

In general, the Koebner phenomenon rash appears 10 to 20 days after the local trauma.3 The reported incidence varies widely from 5% to 75%.5 No definitive pathogenic process has been identified.6 In this patient, the ultraviolet radiation from extensive sun exposure appears to be the trigger. Pathogenic immunologic processes may be enhanced

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